1. Field of the Invention
The present invention relates to a method of operating a fiber-optic acoustical sensor, to an apparatus for use in practicing the method, and to an in-line fiber-optic polarizer usable in such an acoustical sensor. More particularly, the present invention is in the field of method and apparatus for a fiber-optic acoustical sensor used as a hydrophone (i.e., as a microphone used in water to receive sound transmitted through the water), and having particular utility in under-sea seismic exploration.
The present invention also relates to an in-line fiber-optic polarizer. This in-line fiber-optic polarizer is usable in an acoustical sensor embodying the invention.
2. Related Technology
Conventional electrical hydrophones are well known. In seismic exploration, these hydrophones are conventionally employed in static arrays of plural acoustical transducers which are placed on or beneath the sea floor, or in towed arrays (i.e., towed in sea water behind a transport ship or boat). The portion of the acoustical sensor immersed in sea water is generally referred to as the "wet" portion, while the portion on shore or aboard the transport vessel is the "dry" portion.
The dry portion of the sensor may include signal analyzers, recorders, and display devices, for example. Connecting the wet and dry portions of the acoustical sensor is an elongate cable or cables extending between the acoustical transducers and the dry portion of the sensor. In some cases, the acoustical transducers are simply spaced out along the length of the connecting cable in a linear array located in the distal portion of the cable. This linear array configuration of acoustical transducers is generally used for towed arrays.
In the use of hydrophone arrays for seismic exploration, acoustical energy is provided in the water (by a sounding device or explosion, for example). Sound waves from the energy source travel through the water and penetrate into the earth at the sea floor. The sound waves are reflected from geological structures beneath the sea floor (i.e., from oil shale formations, for example), travel back into the sea water above, and are sensed by the acoustical transducers of the hydrophone array. These acoustic transducers (or hydrophones) thus provide signals indicative of the sound energy recovered from the reflections sensed at particular locations in the static or towed array. In this way, the undersea geological structures can be acoustically detected, and with the collection of sufficient data, can be acoustically "imaged".
Because the recovery of sound energy is required at a multitude of spaced apart locations in order to acoustically image undersea geological structures, conventional hydrophone arrays include a multitude of acoustical transducers. Also, because of the large number of transducers in conventional electrical hydrophone arrays, the conventional electrical arrays have had to include a great number of electrical wires and active electrical circuits in the array itself as well as in the connecting cable or cables. These active electrical circuits include, for example, power distribution circuits, amplifiers, repeaters, multiplexers, and other signal conditioning and interpreting circuits. A result with a conventional electrical hydrophone array is that the hydrophone array itself, as well as the cable(s) connecting the array to the dry portion of the acoustical sensor, has to include a multitude of electrical conductors, is larger and much heavier than is convenient, and is expensive.
A particular disadvantage of these conventional electrical hydrophone arrays has been the presence of active electrical circuits within the array and connecting cable(s). These active electrical circuits require electrical power, thus requiring power distribution conductors and amplifiers in the wet portion of the array. The power distribution circuits and amplifiers utilize high voltages. Accordingly, it has followed inexorably that conventional electrical hydrophone arrays present problems with water leaking into the power distribution circuits and active electrical circuits of the array, possibly causing degraded performance because of increased capacitive coupling within the array, and also possibly causing electrical shorting in the array.
For personnel handling such conventional electrical arrays a potential shock hazard is also always present, and influences handling practices with such arrays. That is, even when the hydrophone array is really turned off, personnel have to treat it as though it were on and as though a shock hazard continuously existed. This precaution is necessary in order to establish and maintain safe handling practices.
Further, and undesirably, conventional electrical hydrophone arrays have involved a considerable expense to fabricate the wet portion of the array. This was the case because of the large number of electrical conductors in the array, the presence of the active electrical circuits, and the necessary attempts (frequently unsuccessful) both to make the array water tight during its lifetime, as well as to also be resistant to electrical malfunction, degraded performance, and shorting in the event that water leakage into the array did occur at some time during its useful life.
These problems with conventional electrical hydrophone arrays have led to the development of fiber-optic hydrophone arrays. These conventional fiber-optic hydrophone arrays use fiber-optic acoustical transducers. Light energy is conducted to and from the fiber-optic acoustical sensors along optical fibers extending in the cable portion of the array. No electrical wires are used in the connecting cable or in the array as in electrical hydrophone arrays. Accordingly, such fiber-optic hydrophone arrays do not include active electrical circuits or power distribution circuits in the wet portion of the array.
FIG. 12 illustrates a conventional architecture for a time division multiplexed (TDM) fiber optic hydrophone array. This architecture is conventionally referred to as a ladder network. In this figure, both the upper and lower lines represent fiber optic conductors. The rungs of the ladder are formed by hydrophones in the form of Mach-Zehnder interferometers (or possibly by Michelson interferometers) responsive to ambient acoustic energy. Between adjacent rungs of the ladder, a coil of the optical fiber provides a light propagation delay element having a period "T". At the distal end of the upper conductor, a light pulse of duration .tau.&lt;T is applied. As this pulse proceeds to the right along the upper conductor, a coupler at each rung diverts a portion of the light energy of the pulse into the interferometer. The interferometer provides to the lower conductor, a pulse of light which is phase discriminated as a function of the ambient acoustic energy. As is illustrated, because of the time delay effected progressively along the length of the upper conductor, the pulses delivered into the lower conductor and arriving at the distal (left) end of this lower conductor are time-division multiplexed relative to one another. In other words, the user of such an architecture can distinguish the signals from each particular one of the successive hydrophones along the length of the ladder array because of the arrival of the light pulse from each hydrophone in the train of pulses returned from the array in response to each input pulse. See, "Fiber-optic Sensors--an introduction for Engineers and Scientists", edited by Eric Udd, John Wiley and Sons, 1990, chapters 10 and 11. Also see, "Fiber-optic Sensors", by T. A. Krohn, Instrument Society of America.
FIG. 13 illustrates another conventional architecture for a frequency division multiplexed (FDM) fiber optic hydrophone array. This architecture is conventionally referred to as a matrix network or topology. In this Figure, two continuous-wave light sources are provided, each modulated at a different frequency. Each light source provides light energy into a sensor, each sensor having a pair of interferometers responsive to ambient acoustic energy. The interferometers provide phase discriminated output signals to two output optical fibers. Viewing FIG. 13, the upper fiber carries two output signals, one from one interferometer of the upper pair and the other from one interferometer of the lower pair. The same is true of the lower output conductor. The signals on each conductor are distinguishable from one another because of their modulation carrier frequency. Thus, this arrangement is one of frequency division multiplexing, or FDM.
Those ordinarily skilled in the pertinent arts will recognize that this FDM matrix array concept can be expanded to an array having more than the four hydrophones seen in FIG. 13. That is, the matrix array may have N light sources each with its own modulation frequency different than the others, and N.sup.2 sensors, with N output conductors each carrying N signals distinguishable from one another by their modulation frequency. FIG. 14 illustrates a generalized FDM matrix array topology for a fiber optic acoustical sensor according to this concept.
Unfortunately, a persistent problem with fiber-optic hydrophone arrays has been the existence of "strum", or low frequency noise in the output signal from the array. This low-frequency "strum" noise has severely impacted the performance of conventional fiber-optic acoustical sensors in the frequency range from less than one Hz to several hundred Hz. In fiber-optic acoustical sensors which include acoustical transducers connected to a light source and to a detector by a length of fiber optic cable, physical manipulation of this cable has a dramatic effect on the output signal noise. For example, in a towed hydrophone array in which a tow cable includes a single-mode optical fiber along which light from a laser light source is transmitted to a fiber-optic acoustical transducer, twisting, bending, and stretching of the tow cable in its noisy environment has a dramatic effect on the polarization state of the light in the optical fiber. In other words, the fiber-optic tow cable itself is in some respect a hydrophone exposed to a noisy environment including low frequency twisting, stretching, and bending occurring within the frequency band of interest. This tow-cable-hydrophone effect in turn affects the level of noise in the optical signal at the output of the acoustical transducer, which is transmitted along a return optical fiber to an optical receiver of the hydrophone array.
Further to the above, several conventional polarizers are well known. One polarizer of a fiber-optic type which is known is generally referred to as "Zing" fiber. This particular type of optical fiber has a non-circular cross sectional configuration which preferentially propagates a selected polarization of light. Another type of polarizer is the so called "bulk" polarizer. A bulk polarizer uses a body of material (usually a crystalline substance) preferentially propagating a particular polarization of light. With some transparent crystalline materials, their regular crystal lattice structure makes them usable as bulk polarizers. Still another type of polarizer is known as a Brewster stack of plates. This so-called Brewster stack uses one or more plates made of a material having an index of refraction differing from air, and defining air-material interfaces at which polarizing reflections and refractions take place according to well understood principles of optics. With a Brewster stack of plates made of glass (index about 1.5), .THETA..sub.B, the Brewster angle for polarizing incidence is defined as: .THETA..sub.B =arctan nH/nL, in which n.sub.H and n.sub.L are the index of refraction of the material and of air (index 1). .THETA..sub.B for glass plates is about 55.degree. to 57.degree.. "p" polarized light is transmitted at the end interface without loss, whereas "s" polarized light is largely reflected. Unfortunately, not one of these conventional polarizers has a character which makes it inexpensive, rugged, small in size, conveniently used with optical fiber connection both on the light supply and polarized light delivery side of the polarizer, and offers very good extinction of an undesired polarization state in the polarized light delivered from such a polarizer.